This invention relates to the operation of a fractional distillation column in such a way as to prevent the undesirable condensation of AlCl.sub.3 therein. More particularly, this invention relates to the operation of a fractional distillation column in such manner in a process for separating AlCl.sub.3 from a mixture containing metal chlorides.
As used herein, in accordance with common trade practice, the term "metal chloride" includes silicon tetrachloride (SiCl.sub.4).
In a process for the production of metal chlorides by the chlorination of a material containing metal oxides, a mixture of chlorides and non-condensable gases will be obtained. For example, in a process for the production of AlCl.sub.3 by the chlorination of kaolin clay in the presence of a carbonaceous reducing agent, a mixture of metal chlorides, including AlCl.sub.3, FeCl.sub.3, SiCl.sub.4 and TiCl.sub.4 will be obtained. Also obtained will be various non-condensable gases (at the temperatures and pressures encountered in the chlorination system) such as carbon monoxide (CO), carbon dioxide (CO.sub.2) and chlorine (Cl.sub.2), chlorides such as hydrogen chloride (HCl) and phosgene (COCl.sub.2), and small amounts of other metal chlorides such as calcium chloride (CaCl.sub.2) and sodium chloride (NaCl). Nitrogen (N.sub.2) may be added as a purge gas to the process; consequently, the mixture of chlorides and non-condensable gases obtained may also include N.sub.2 (a non-condensable gas at the temperatures and pressures encountered in the system).
Various processes for the separation of metal chlorides from a mixture of chlorides and non-condensable gases are known. Several of these processes involve the selective condensation or fractional distillation of particular metal chlorides from the mixture. Thus, for example, U.S. Pat. No. 2,387,228 of Arnold describes a two-stage fractional distillation process for the separation of AlCl.sub.3 from a mixture containing AlCl.sub.3, FeCl.sub.3, SiCl.sub.4 and TiCl.sub.4. According to this process, the mixture is introduced into a first distillation column, which is operated at temperatures and pressures sufficient to separate the mixture into a gaseous fraction, which consists mainly of SiCl.sub.4 and TiCl.sub.4, and a liquid fraction, which consists mainly of AlCl.sub.3 and FeCl.sub.3. The liquid fraction from the first column is then introduced into a second distillation column, which is operated at temperatures and pressures sufficient to separate AlCl.sub.3 in the gaseous state from the liquid fraction.
Another process for the separation of AlCl.sub.3 from a mixture containing AlCl.sub.3 and other metal chlorides is disclosed in the commonly owned, copending application of Stewart, entitled "Fractional Distillation Process For The Production of Aluminum Chloride", Ser. No. 224,280, filed Jan. 12, 1981. According to this process, the mixture is introduced into a first distillation column, which is operated at temperatures and pressures sufficient to separate FeCl.sub.3 from the mixture, while avoiding the formation of a solid solution of FeCl.sub.3 and AlCl.sub.3 within the column. At least one additional distillation column is then utilized to remove AlCl.sub.3 from the remaining mixture of metal chlorides obtained from the first column.
Other known processes for the fractional distillation or selective condensation of particular metal chlorides from a mixture of chlorides include that of U.S. Pat. No. 3,436,211 of Dewing and that of U.S. Pat. No. 3,786,135 of King et al. The Dewing process operates to remove calcium chloride (CaCl.sub.2) and magnesium chloride (MgCl.sub.2) from a gaseous mixture containing these chlorides and AlCl.sub.3, and the King process operates to selectively condense sodium aluminum chloride (NaAlCl.sub.4) from the gaseous effluent derived from the chlorination of alumina which is contaminated with sodium.
Unfortunately, however, problems have arisen in many instances where a fractional distillation or selective condensation procedure has been utilized to separate a particular fraction from a mixture of metal chlorides. Such problems may arise because the range of temperatures in distillation columns operated at convenient pressures for the separation of particular metal chlorides from the mixture may include temperatures at which undesirable condensation of other metal chlorides takes place. Thus, for example, as described in the aforementioned application of Stewart, Ser. No. 224,280, the formation of a solid solution of AlCl.sub.3 and FeCl.sub.3 may interfere with the operation of a fractional distillation process for the separation of AlCl.sub.3 from a mixture containing AlCl.sub.3 and other chlorides including FeCl.sub.3. In the process of Stewart, the undesirable condensation of AlCl.sub.3 and FeCl.sub.3 in the form of a solid solution is avoided by operating the first distillation column at temperatures and pressures sufficient to separate FeCl.sub.3 from the mixture while avoiding the formation of the undesirable solid solution.
A similar problem is discussed in U.S. Pat. No. 2,718,279 of Kraus, which describes a fractional condensation process involving the separation of FeCl.sub.3 from a mixture of gases including FeCl.sub.3 and TiCl.sub.4. According to this reference, fractional condensation of such a mixture is hindered by the tendency of FeCl.sub.3 to condense or precipitate on the interior surfaces of conventional condensing apparatus which are operated at convenient temperatures and pressures for the separation of TiCl.sub.4 from the mixture. According to the method of Kraus, this problem is overcome by the washing of the FeCl.sub.3 from the gaseous mixture with a continuous flow of liquid TiCl.sub.4.
U.S. Pat. No. 2,870,869 of Mahler describes a problem which has accompanied the separation of TiCl.sub.4 from a gaseous mixture containing TiCl.sub.4 and other metal chlorides, including AlCl.sub.3 and FeCl.sub.3. According to this reference, the condensation of TiCl.sub.4 from such a mixture may be accompanied by the formation of a considerable amount of finely divided solid particles of AlCl.sub.3 and FeCl.sub.3. According to the process of Mahler, this problem is solved by the provision of a chamber containing a slurry of solid AlCl.sub.3 and FeCl.sub.3 suspended in liquid TiCl.sub.4. The slurry is maintained at a temperature close to but below the dew point of TiCl.sub.4 and is agitated within the chamber so as to provide a turbulent shower therein. The gaseous mixture is then passed through the chamber and the gaseous AlCl.sub.3 and FeCl.sub.3 in the mixture are reportedly washed therefrom by contact with the turbulent shower.
Other processes for the separation of a particular fraction from a mixture containing metal chlorides have attempted to solve the problem of undesirable condensation of particular constituents in the mixture by employment of liquid chloride solvents or wash media. For example, U.S. Pat. No. 2,533,021 to Krchma describes the separation of FeCl.sub.3 from a mixture of gases by dissolving the FeCl.sub.3 in a solvent consisting of a mixture of molten FeCl.sub.3 and NaCl. Similarly, U.S. Pat. No. 3,294,482 to Lerner describes the separation of FeCl.sub.3 from a gaseous mixture of metal chlorides which contains the chlorides of iron, columbium, tungsten, molybdenum, and zirconium, by scrubbing the gaseous mixture with a molten mixture of FeCl.sub.3 and NaCl.
U.S. Pat. Nos. 3,938,969 and 4,035,169 of Sebenik et al. refer to a method for separating AlCl.sub.3 from a mixture of solids containing FeCl.sub.3. According to this method, the mixture is washed with TiCl.sub.4 to dissolve the AlCl.sub.3, leaving a suspension of solid particles of FeCl.sub.3 in a solution of liquid AlCl.sub.3 and TiCl.sub.4. According to the Sebenik patents, the solubility of AlCl.sub.3 in TiCl.sub.4 is relatively low; consequently, relatively large quantities of TiCl.sub.4 are required to dissolve a quantity of AlCl.sub.3, thereby making this method impractical. The Sebenik patents also describe a method by which AlCl.sub.3 may be separated from a mixture of gaseous metal chlorides by washing the mixture with a solvent which preferentially dissolves AlCl.sub.3 and which may dissolve FeCl.sub.3, while failing to dissolve, or at most dissolving only sparingly other chlorides such as SiCl.sub.4 and TiCl.sub.4.
Another use for the application of a liquid metal chloride to a gaseous mixture of chlorides is discussed by Dr. Robert Powell on page 91 of a 1968 publication of the Noyes Development Corporation, entitled "Titanium Dioxide and Titanium Tetrachloride". He describes the evaporation of liquid TiCl.sub.4 in a stream of chlorination gases to cool the stream.